Phase locked loop comprising a sigma-delta modulator

ABSTRACT

The invention is directed to a phase locked loop with a ΣΔ modulator. A multimodulus divider in the feedback path of the PLL is actuated by the ΣΔ modulator. The latter has a design which can be described by a complex transfer function H(s) in the Laplace plane, said transfer function having a complex-conjugate pair of pole points. The arrangement allows a significant reduction in the noise in critical frequency domains and hence allows adherence to transmission masks based on radio specification even when the PLL bandwidth is as large as the modulation bandwidth.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/DE03/03894, filed Nov. 25,2003 which was not published in English, that claims the benefit of thepriority date of German Patent Application No. DE 102 57 185.6, filed onDec. 6, 2002, the contents of which both are herein incorporated byreference in their entireties.

FIELD OF THE INVENTION

The present invention relates to a phase locked loop with a ΣΔ(sigma-delta) modulator.

BACKGROUND OF THE INVENTION

A phase locked loop (PLL) of the generic type is specified in theprinted document U.S. Pat. No. 6,008,703. This document describes afrequency synthesizer in which a ΣΔ PLL is used to produce a signal at amodulated carrier frequency. This phase locked loop, which is shown byway of example in FIG. 2A in the indicated prior art document, comprisesa forward path with a phase detector, a loop filter and a controlledoscillator and a feedback path which couples the output of theoscillator to the phase detector via a multimodulus divider. To controlthe multimodulus divider, a digital sigma-delta modulator is providedwhose input side is supplied both with information about the desiredcarrier signal and with digital modulation data. The modulated outputsignal is provided at the output of the controlled oscillator.

In a phase locked loop of the generic type, the desired frequencymodulation is performed digitally by varying the frequency divisionvalue. Such frequency synthesizers are used, by way of example, inmodern, digital radio systems for carrier frequency generation and fordigital frequency modulation.

When stipulating the dimensions of or designing such a phase lockedloop, the choice of bandwidth for the phase locked loop is ofparticularly great importance. In this context, it is necessary to finda compromise between the noise properties of the circuit and themodulation bandwidth. On the one hand, the noise needs to be as low aspossible in order to adhere to the spectral transmission masksprescribed in the various radio specifications. This requires theselection of a relatively small loop bandwidth. On the other hand, thisis opposed by the fact that transmitting modulated data requires a largebandwidth for modern applications in communications technology.

By way of example, the European Telecommunications Standard “ETSI EN 300175-2 V1.5.1 (2001-02) DECT (Digital Enhanced CordlessTelecommunications) CI (Common Interface) PART 2: Physical Layer”specifies limits for unwanted radio emissions. The maximum power levelfor the third and fourth adjacent channels is limited to 80 nW and 40nW, respectively, for example, see section 5.5, page 24.

A system-related, dominant noise component arises through thequantization noise of the ΣΔ modulator itself. The ΣΔ modulator normallyactuates the multimodulus divider and in so doing brings about randomchangeover between integer division ratios in order to obtain on averagethat division ratio which brings about generation of the desired outputfrequency from the PLL.

FIG. 10A of the prior art document U.S. Pat. No. 6,008,703 (which wascited earlier herein) shows a multimodulus divider which comprises aseries circuit containing a plurality of frequency divider stages. Inthis case, the frequency divider stages can each be changed over betweenprecisely two division values and can divide the input frequency eitherby the division value 2 or by the division value 3. Such frequencydivider stages are also called ⅔ frequency dividers. The range ofdivision values which can be set for such a multimodulus divider, whichis constructed only from ⅔ divider stages, is determined in line withthe following specification:$N = {N_{0} + {\sum\limits_{i = 0}^{L - 1}\quad{c_{i} \cdot 2^{i}}}}$where L denotes the number of ⅔ divider stages and N₀ is equal to 2^(L).

The ΣΔ modulator actuating such a multimodulus divider is normallyimplemented in a “MASH” architecture. A stage in such a multistage MASHmodulator is shown by way of example in FIG. 8B of the prior artdocument U.S. Pat. No. 6,008,703. In this case, a summing element isprovided which has two inputs and an output, with an error signal beingfed back by means of a feedback path. In this arrangement, the errorsignal is normally fed back with a delay. A multistage ΣΔ modulator withMASH architecture is illustrated in FIG. 8A with the aid of a blockdiagram.

The ΣΔ modulator distributes the power of the quantization noise in linewith its noise transfer function over the frequency band. The noisetransfer function (NTF) of the MASH structure can be described by theequationNTZ(z)=(1−z ⁻¹)Nwhere N represents the order of the modulator. The quantization noise isin this case shifted from lower frequencies to higher frequencies. Theactuation of the multimodulus divider by the ΣΔ modulator brings aboutrandom changeover of the division values in line with the spectraldistribution of the quantization noise. The random changeover of thedivision factor in turn brings about a change in the frequency over timeand thus produces a “frequency or phase interference swing” (FM or PMinterference swing). This is also called residual FM jitter in theliterature, or else phase noise. The magnitude of this interferenceswing determines the signal-to-noise ratio (SNR) of thefrequency-modulated or phase-modulated carrier signal and therefore hassignificant effects on the performance of a radio receiver of suchdesign, for example on its range.

In the case of the frequency synthesizer based on U.S. Pat. No.6,008,703, the explained noise demands mean that the bandwidth of thephase locked loop is made significantly smaller than is actuallyrequired for transmitting the modulated data. To compensate for theresultant frequency response of the loop filter in the PLL, the data tobe modulated are subjected to digital precompensation before beingsupplied to the ΣΔ modulator. This involves high frequency componentsbeing raised digitally.

A fundamental drawback of this design is the very accurate matchingwhich is required between the digital compensation filter, on the onehand, and the analog loop filter, on the other. This is because if thebandwidth of the control loop changes as a result of analog influencessuch as manufacturing tolerances, temperature drifts or ageingphenomena, the digital precompensation (which is not subject to theseinfluences in practice) raises the high frequency components too much ortoo little.

Another option for compensating for a reduced loop bandwidth is providedby “two-point modulation”. This involves modulation at two modulationpoints in the phase locked loop, first on the frequency divider andsecondly at the oscillator input. In this case, one of the twomodulation points of the PLL has low-pass filter properties and theother has high-pass filter properties. The overall result is therefore aconstant transfer function for the modulation data. In this case too,however, the problem of accurate matching being required between theanalog and digital signal paths (which problem has already beenexplained) arises.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the invention. This summary isnot an extensive overview of the invention, and is neither intended toidentify key or critical elements of the invention, nor to delineate thescope thereof. Rather, the primary purpose of the summary is to presentone or more concepts of the invention in a simplified form as a preludeto the more detailed description that is presented later.

The present invention is directed to a phase locked loop with a ΣΔmodulator which affords a large modulation bandwidth, allows adherenceto spectral transmission masks as provided in the mobile radiostandards, and avoids the problem of mismatches between analog anddigital circuit parts.

The invention comprises a phase locked loop with a ΣΔ modulator thatcomprises a feedback path which couples the output of a summing elementto an input thereof and which is designed such that the complex transferfunction representing the feedback path has a complex, nonzero pair ofpole points in the Laplace plane.

In this case, the complex transfer function representing the feedbackpath may also contain a plurality of complex, nonzero pairs of polepoints in the Laplace plane.

The present invention is based on the “error feedback topology” of a ΣΔarchitecture. In comparison with a MASH structure, which in line withits transfer function in the Laplace planeH(s)=1/s ³has a triple pole point at s=0, the present invention involvesimplementing a transfer function with at least one complex-conjugatepair of pole points. Suitable selection of the imaginary and real partsof the complex pair of pole points means that there is no difficulty inensuring adherence to the spectral transmission masks required in radiospecifications.

The complex transfer function H(s) in the Laplace plane is used todetermine the associated noise transfer function asNTF(z)=1−H(z).

The complex pole pair provided in accordance with the present invention,which pole pair differs from zero, makes it possible to reduce the noisein a critical frequency range. In this case, the imaginary part of theadditional, complex pair of pole points determines the center frequencyerror, while the real part stipulates the suppression of the noise forthis center frequency error.

The architecture of the present invention makes it possible to stipulatethe dimensions of a noise transfer function, which can be optimized forthe spectral transmission mask specifically by taking into account therequirements.

In the DECT (Digital Enhanced Cordless Telecommunication) system, forexample, the demands on the spectral power transmission in the frequencyinterval at 4 MHz to 8 MHz are normally critical for adherence, thisbeing in line with the third and fourth adjacent channels, since firstlythe ΣΔ quantization noise still rises greatly and is not sufficientlyattenuated by the loop filter, and secondly the transmitted power in theadjacent channel may be only 80 or 40 nW, in line with the third andfourth adjacent channels. This is stipulated in the ETSI standard forthe physical plane in the DECT standard.

In accordance with the present invention, transmission arrangements withthe phase locked loop can be provided for generating frequency-modulatedor phase-modulated signals. Alternatively, the phase locked loopdescribed can be used in a frequency synthesizer for providing a carrierfrequency.

The described invention is used to transmit the full modulationbandwidth via the phase locked loop. In this context, the bandwidth ofthe phase locked loop is advantageously as large as the modulationbandwidth. It is thus possible for the high matching demands of knowntransmission designs between the analog phase locked loop and thedigital signal processing part of the ΣΔ PLL to be circumvented.

To allow the large phase locked loop bandwidth, in summary, a ΣΔmodulator is used taking into account the spectral transmission mask. Tothis end, as explained, the present invention involves a complex zeropoint being inserted in the noise transfer function in the criticaladjacent channel, said zero point resulting in adherence to thetransmission mask despite the larger bandwidth.

In accordance with one embodiment of the invention, the complex transferfunction H(s) described on the basis of the Laplace variable s has notonly the complex pair of pole points but also a double pole point at thevalue s=0.

By way of example, the complex transfer function of the feedback pathpreferably satisfies the equation${H\quad(s)} = \frac{1}{s^{2} \cdot \left( {s + \sigma_{1} + {j\quad\omega_{1}}} \right) \cdot \left( {s + \sigma_{2} - {j\quad\omega_{2}}} \right)}$or more generally:${H\quad(s)} = \frac{1}{\prod\limits_{i = 0}^{n}\quad\left( {s + {\sigma_{i} \pm {j\quad\omega_{i}}}} \right)}$where σ_(i) is less than or equal to zero and at least one σ_(i) differsfrom zero.

In accordance with another embodiment of the present invention, thefrequency divider is in the form of a “multimodulus divider”. By way ofexample, the multimodulus divider may have a range of adjustabledivision values from 32 to 63. This allows a further frequency range tobe covered by the phase locked loop with a modulator.

In accordance with one embodiment of the present invention, a furtherfrequency divider is provided at the output of the controlledoscillator. In this arrangement, the further frequency divider has aplurality of outputs which each provide mutually phase-shifted signals.

In this case, the output of the phase locked loop is formed at theoutput of the further frequency divider, not directly at the output ofthe controlled oscillator. The oscillator oscillates at a higherfrequency corresponding to the frequency division ratio of the furtherfrequency divider.

In addition, the invention, in one example, includes a multiplexer witha plurality of inputs. In this case, each output of the furtherfrequency divider has an associated input to the multiplexer, and isconnected thereto. The output of the multiplexer is connected to theinput of the multimodulus divider.

In accordance with another embodiment, the multimodulus divider issupplied with the output signal from the phase locked loop with aselectable phase shift. This brings about a reduction in the dividerstepsize ΔN to values of less than 1 relative to the output frequency,which means that the quantization noise is distributed over a smallerfrequency range and thus results in a reduced interference swing. Thisresults in a further improvement in the options for adhering to thespectral transmission mask of a mobile radio standard.

The control input of the multiplexer is, in one example, connected tothe ΣΔ modulator for preselecting the phase shift.

The further frequency divider comprises in one example a :2 frequencydivider. In this case, the frequency of the signal provided at theoutput of the controlled oscillator corresponds to twice the outputfrequency of the phase locked loop.

In one example, the further frequency divider has four outputs whichprovide the signal at the output frequency of the phase locked loopafter phase shifting in steps of 90° relative to one another. Thisreduces the divider stepsize ΔN from 1 to 0.25 relative to the outputfrequency of the PLL.

In accordance with yet another embodiment of the invention, themultimodulus divider comprises a series circuit containing a pluralityof ⅔ frequency divider stages, which can be changed over between afrequency division ratio of 2 and a frequency division ratio of 3,respectively. In this case, the frequency division ratio of 2corresponds to a halving of the frequency. The frequency division ratioof 3 means that the input signal frequency is divided by 3.

The phase locked loop contains in one example a charge pump circuit witha downstream loop filter in order to couple the phase comparator to thecontrolled oscillator. In this case, the loop filter is designed for aloop bandwidth which is the same as the modulation bandwidth.

To the accomplishment of the foregoing and related ends, the inventioncomprises the features hereinafter fully described and particularlypointed out in the claims. The following description and the annexeddrawings set forth in detail certain illustrative aspects andimplementations of the invention. These are indicative, however, of buta few of the various ways in which the principles of the invention maybe employed. Other objects, advantages and novel features of theinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail using exemplary embodimentswith reference to the drawings, in which:

FIG. 1 is a block diagram illustrating a phase locked loop with ΣΔmodulation in one exemplary embodiment of the invention,

FIG. 2 is a graph illustrating the transmission mask in accordance withthe DECT specification and the frequency profile in accordance with theinvention and the prior art, respectively, with the spectral powerdensity being plotted against frequency, and

FIG. 3 is a block diagram illustrating an exemplary embodiment of a4th-order ΣΔ modulator as shown in FIG. 1 with a noise transfer functionas shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a phase locked loop with a phase comparator 1 having twoinputs and an output. One of the two inputs of the phase detector orphase comparator 1 can have a reference frequency generator (not shownhere) connected to it which delivers a reference frequency F_(ref). Theoutput of the phase detector 1 has a voltage controlled oscillator 4connected to it via a charge pump circuit 2 and a loop filter 3, whichis in the form of a low-pass filter and which is connected downstream ofthe charge pump circuit 2. The output of the oscillator 4 has a 4:1multiplexer 6 connected to it via a :2 frequency divider 5. The :2frequency divider 5 has four outputs at which it is possible to tap offthe signal at the output frequency F_(out) of the phase locked loop infour different phases. One of the four outputs of the frequency divider5 forms the output 8 of the phase locked loop via an amplifier 7.

The four outputs of the frequency divider 5 are connected to arespective associated input of the multiplexer 6, which is arranged in afeedback path of the phase locked loop. The output of the multiplexer 6is connected to a further input of the phase detector 1 via amultimodulus divider 9. The multimodulus divider 9 comprises in thisexample a series circuit containing a total of five frequency dividerstages. Each frequency divider stage is designed as a ⅔ frequencydivider, in which the frequency division value can be changed overbetween 2 and 3. The control inputs of the frequency divider stages arecombined to form a programming input 10 for the multimodulus divider 9.At the programming input 10, it is possible to set the desired frequencydivision value of the multimodulus divider in a range N between 32 and63. The programming input 10 is connected to the output of a ΣΔmodulator 11. The output of the ΣΔ modulator 11 also has the controlinput of the multiplexer 6 connected to it via a control unit 12.

The ΣΔ modulator 11 comprises a first input 13 for supplying a channelword, that is to say information about the phase locked loop's channelwhich is to be set. A further input 14 is used to supply the digitalmodulation data. The two inputs 13, 14 are connected to one another andto a feedback path in a summing element 15. The feedback path returnsthe output of the summing element 15 to an input of the summing elementvia a finite impulse response (FIR) filter 16. The FIR filter 16 has atransfer function H(z) which is complex. In the Laplace plane, thetransfer function H(s) can be described, through transformation, by theformula${H\quad(s)} = \frac{1}{s^{2} \cdot \left( {s + \sigma_{1} + {j\quad\omega_{1}}} \right) \cdot \left( {s + \sigma_{2} - {j\quad\omega_{2}}} \right)}$

The noise transfer function NTF of the ΣΔ modulator 11 is in this casedetermined in the z plane from NTF(z)=1−H(z).

In the ΣΔ fractional N PLL of the present invention, the oscillator 4oscillates at twice the output frequency f_(out). The oscillator 4 iscontrolled by a comparison result for a reference frequency f_(ref) andthe divided-down frequency f_(div) provided by the multimodulus divider9. In the phase locked loop's forward path 1, 2, 3, 4, 5 there is anadditional frequency divider 5. This provides the output frequency infour different phases between which it is possible to select using themultiplexer 6. With reference to the output frequency f_(out) from thePLL, a stepsize ΔN referring to the division ratio is thus producedwhich is less than 1. The proposed PLL allows changeover not only infrequency steps which are prescribed by the reference frequency f_(ref),in the present case 40 MHz, but also in stepsizes of 10 MHz which aregoverned by the factor 0.25.

The smaller stepsize means that the instantaneous frequency varies overa smaller frequency range, so that the quantization noise is alsoreduced by the factor of the division step reduction. When fourdifferent phases are used, as in the present case, the interferenceswing is reduced by the factor 4, which corresponds to a spectralreduction in the phase noise of 12 dB. The two least significant bits ofthe ΣΔ modulator 11 are used to actuate the multiplexer 6 via thecontrol unit 12. Five further control bits in the ΣΔ modulator are usedto program the division value of the multimodulus divider 9.

In comparison with a conventional MASH structure, whose transferfunction in the Laplace plane is a triple pole point at s=0, the ΣΔmodulator of the present invention contains a chosen complex-conjugatepair of pole pointss _(1,2)=−σ1±jω1such that the pole point is at that frequency or in that frequency rangewhich needs to be reduced on account of the frequency mask that is to beadhered to.

The inserted complex pole point of the present invention significantlyreduces the noise in the critical frequency range. In this case, theimaginary part ω1 of the complex-conjugate pole pair s_(1,2) determinesthe frequency to be reduced, while the real part −σ1 stipulates themeasure of suppression at this frequency.

The present invention relates to a transmitter design with a high levelof spectral purity which can be used either to generatefrequency-modulated and/or phase-modulated signals or else as afrequency synthesizer. Since the invention allows the full modulationbandwidth to be transmitted via the phase locked loop, a ΣΔ modulator 11with the noise transfer function NTF(z)=1−H(z) is used taking intoaccount the spectral transmission mask that is to be adhered to. To thisend, as proposed, a complex zero point is inserted into the noisetransfer function in the critical adjacent channel, and this results inadherence to the transmission mask despite the larger bandwidth which isavailable.

In the case of the exemplary embodiment shown in FIG. 1, the phaselocked loop with a ΣΔ modulator is designed such that it can be used forthe DECT system in a frequency band from 1880 MHz to 1900 MHz and at thesame time for WDCT in the ISM band in a frequency range from 2.40 to2.48 GHz. To this end, it is possible to set division values between 32and 63.75 with a stepsize of 0.25.

To cover the two frequency bands, a dual band extension of the PLL mayeither contain a second VCO, or the oscillator 4 can be equipped with aswitchable frequency band.

The loop bandwidth is advantageously so large that modulation via thedata input 14 is possible.

FIG. 2 uses a semilogarithmic graph to show the spectral power densityin dBc per Hertz plotted against the relative center frequency error inHertz. The transmission mask to be adhered to in line with the DECTspecification in the present example is shown by means of a curve andhas been provided with the reference symbol 20. The frequency profilewhich can be obtained, as described at the outset, using a conventionalthird-order ΣΔ-MASH modulator has been provided with the referencesymbol 30. It can be seen that the DECT mask cannot be adhered to inoptimum fashion in a frequency range between 10⁶ and 10⁷ Hz. Referencesymbol 40 denotes the profile of the noise transfer function which canbe attained using the arrangement of the present invention shown in FIG.1.

The advantageous effect of the complex-conjugate pole point insertedinto the modulator's feedback path in line with the invention canclearly be seen in FIG. 2. This pole point causes a significantreduction in the transmitted spectral noise power to be attainedprecisely in the critical frequency range. The DECT mask can thus beadhered to without a problem.

FIG. 3 is a block diagram illustrating an exemplary embodiment of the ΣΔmodulator 11 from FIG. 1, which is implemented in line with the equation${H\quad(s)} = \frac{1}{s^{2} \cdot \left( {s + \sigma_{1} + {j\quad\omega_{1}}} \right) \cdot \left( {s + \sigma_{2} - {j\quad\omega_{2}}} \right)}$that is valid in the Laplace plane.

The 4th-order ΣΔ modulator shown in FIG. 3 has a signal input 41, asignal output 42 and an error feedback input 43 which is connected tothe output 42.

The input 41 has a first summing element 44 connected to it with threeinputs and an output. The output of the summing element 44 is connectedvia a 1/z element 45 and a second summing element 46 to a signal block47 which shifts the digital signal two bits to the right. The output ofthe block 47 forms the output 42 of the modulator. The error signalfeedback input 43 is connected to a node K via a 1/z element 48. Asignal path leads from the node K via a multiplier with the factor K0,which is provided with reference symbol 49, to a further input of thesecond summing element 46. In addition, the node K is connected to aninput of the first summing element 44 via a multiplying element 50,which multiplies by the factor K1. The node K is also connected to aninput of the summator 44 via a 1/z element 51 and via a multiplier 52which multiplies by the factor K2. Finally, the output of the 1/zelement 51 is connected to the first summing element 44 via a further1/z element 53 and via a multiplier which operates with the factor K3and has the reference symbol 54. The signal diagram shown in FIG. 3which has been described describes a ΣΔ modulator 11 whose attainednoise transfer function corresponds to the profile 40 from FIG. 2 on thebasis of the inserted complex-conjugate pole point.

While the invention has been illustrated and described with respect toone or more implementations, alterations and/or modifications may bemade to the illustrated examples without departing from the spirit andscope of the appended claims. In particular regard to the variousfunctions performed by the above described components or structures(assemblies, devices, circuits, systems, etc.), the terms (including areference to a “means”) used to describe such components are intended tocorrespond, unless otherwise indicated, to any component or structurewhich performs the specified function of the described component (e.g.,that is functionally equivalent), even though not structurallyequivalent to the disclosed structure which performs the function in theherein illustrated exemplary implementations of the invention. Inaddition, while a particular feature of the invention may have beendisclosed with respect to only one of several implementations, suchfeature may be combined with one or more other features of the otherimplementations as may be desired and advantageous for any given orparticular application. Furthermore, to the extent that the terms“including”, “includes”, “having”, “has”, “with”, or variants thereofare used in either the detailed description and the claims, such termsare intended to be inclusive in a manner similar to the term“comprising”.

1. A phase locked loop, comprising: a phase comparator comprising twoinputs and an output; a controlled oscillator comprising a control inputcoupled to the output of the phase comparator; a frequency dividercomprising a signal input coupled to an output of the controlledoscillator, an output, and a control input for preselecting a divisionratio associated therewith; and a sigma-delta modulator connected to thecontrol input of the frequency divider, wherein the sigma-deltamodulator comprises a feedback path that couples an output of a summingelement to an input thereof, and is configured having a complex transferfunction representing the feedback path comprising at least one complex,nonzero pair of pole points in the Laplace plane.
 2. The phase lockedloop of claim 1, wherein the complex transfer function of thesigma-delta modulator is described on the basis of the Laplace variables, and further comprises at least one pole point at the value s=0. 3.The phase locked loop of claim 1, wherein the complex transfer functionof the feedback path of the sigma-delta modulator satisfies the equation${H\quad(s)} = \frac{1}{\prod\limits_{i = 0}^{n}\quad\left( {s + {\sigma_{i} \pm {j\quad\omega_{i}}}} \right)}$where Σ_(i) is less than or equal to zero and at least one σ_(i) differsfrom zero.
 4. The phase locked loop of claim 1, wherein the frequencydivider comprises a multimodulus divider.
 5. The phase locked loop ofclaim 4, further comprising: a further frequency divider coupled to theoutput of the controlled oscillator, the further frequency dividercomprising a plurality of outputs that each provide divided, mutuallyphase-shifted signals; and a multiplexer comprising a plurality ofinputs that are each associated with and connected to the outputs of thefurther frequency divider, with an output connected to the input of themultimodulus divider, and with a control input.
 6. The phase locked loopof claim 5, wherein the further frequency divider comprises adivision-by-two frequency divider such that the frequency of a signalprovided at the output of the controlled oscillator corresponds to twicethe output frequency at the output of the further frequency divider. 7.The phase locked loop of claim 5, wherein the further frequency dividercomprises four outputs, and is configured to provide four signals thatare phase-shifted in steps of 90 degrees relative to one another, as theoutput frequency of the phase locked loop.
 8. The phase locked loop ofclaim 4, wherein the multimodulus divider comprises a series circuitcontaining a plurality of ⅔ frequency divider stages that are configuredto change over between a frequency division ratio of two and a frequencydivision ratio of three.
 9. The phase locked loop of claim 1, furthercomprising a series circuit comprising a charge pump circuit and a loopfilter coupled between the phase comparator and the control input of thecontrolled oscillator.
 10. A phase locked loop, comprising: a phasecomparator configured to compare a reference frequency signal with adivided feedback signal, and generate a control signal in responsethereto; a controlled oscillator configured to generate a controlledfrequency signal as a function of the control signal; a programmablefrequency divider circuit configured to receive a feedback signalassociated with the controlled frequency signal and generate the dividedfeedback signal by dividing the feedback signal by a value that is basedon a division control signal coupled thereto; and a sigma-deltamodulator configured to generate the division control signal based onmodulation data supplied thereto, the sigma-delta converter comprisingan internal control loop having a feedback path, wherein a complextransfer function of the feedback path comprises at least one complex,nonzero pair of pole points in the Laplace plane.
 11. The phase lockedloop of claim 10, wherein the complex transfer function of the feedbackpath in the sigma-delta modulator further comprises at least one furtherpole point at the value s=0 in the Laplace plane.
 12. The phase lockedloop of claim 10, wherein the complex transfer function of the feedbackpath in the sigma-delta modulator satisfies the equation:H(s)=1/[s ²(s+σ ₁ +jω ₁)(s+σ ₁ −jω ₁)], wherein ω₁ represents afrequency to be suppressed, and σ₁ represents an amount of suppressionat the frequency.
 13. The phase locked loop of claim 10, furthercomprising: A further frequency divider circuit configured to receivethe controlled frequency signal from the controlled oscillator andoutput a plurality of divided down controlled frequency signals eachhaving differing, varied phases with respect to one another, and whereinan output of the phase locked loop is taken from one of the divided downcontrolled frequency signals; and A multiplexer configured to receivethe plurality of divided down controlled frequency signals andselectively pass one of the signals to the programmable frequencydivider circuit based on a selection signal provided by the sigma-deltaconverter.
 14. The phase locked loop of claim 13, wherein thesigma-delta modulator is further configured to generate the selectionsignal provided to the multiplexer based on the division control signal.15. The phase locked loop of claim 13, wherein the plurality of divideddown control frequency signals are consecutively ninety degrees out ofphase with one another.
 16. The phase locked loop of claim 10, furthercomprising a series circuit comprising a charge pump and a loop filterconfigured to generate a tuning signal for the controlled oscillatorbased on the control signal from the phase comparator.